Chapter 19: Global Change in the Earth System
Feature Articles
1: The Melting of Snowball Earth
The Melting of Snowball Earth
by Stephen Marshak
Researchers Schrag and Hoffman point to a rock layer that represents the abrupt end of a 700-million-year-old event. The light-colored boulder in the rock between them probably once traveled with an iceberg and fell to the muddy seafloor when the ice melted.
Credit: Daniel P. Schrag
Recently discovered evidence suggests that at least twice during the Proterozoic eon (once at 2.2 Ga and again at about 0.7 Ga), the Earth nearly froze over and became a nearly lifeless snowball. This evidence came from fieldwork by geologists studying strata in Canada, Siberia, South Africa, and Australia. Geologists noticed layers of sediment that had been deposited by glaciers was in a sequence of 2.2-Ga strata and in a sequence of 0.7-Ga strata. These strata are sandwiched between limestone layers. Paleomagnetic data, which can be used to indicate the latitude at which sediment accumulated, indicated that the glacial sediments formed at the equator. And since the glacial strata form in seawater, they must have accumulated near sea level. Putting this information together, geologists concluded that glacial conditions occurred at sea level at the equator at both 2.2 and 0.7 Ga—thus, the entire Earth must have been cold enough for snow. In some models of this time, a layer of ice over a kilometer thick covered the oceans. The sandwiching of glacial strata between limestone beds suggests that the episodes of global glaciation must have been short-lived.
Researchers have determined that for this condition, called snowball Earth, to exist, either the heat production of the Sun, or the concentration of greenhouse gases like CO2 in the atmosphere must decrease. But once the processes of freezing starts, positive feedback caused by the albedo of ice would cool the Earth so much that freezing would take place across the planet. When positive feedback relentlessly drives a system in one direction, we have a "runaway" situation—"runaway ice albedo" may have caused snowball Earth. Calculations show that if the Earth were a snowball, its albedo would be so great that unless some other factor in the environment changed, the climate could never warm enough to melt the ice.
So how does the Earth recover from a snowball state? Researchers speculate that once ice covered the Earth, the oceans could not absorb CO2 (normally CO2 in the atmosphere dissolves in the sea, where it becomes incorporated in plankton shells and thus eventually in limestone). In addition, chemical weathering of rock effectively came to a halt, because almost all rock was protected from the atmosphere by ice. (Chemical-weathering reactions absorb CO2). Without the sea to dissolve CO2, and without weathering reactions to absorb CO2, the carbon dioxide emitted into the atmosphere by volcanoes would build up. After a few million years, enough of this greenhouse gas accumulated that the climate warmed and ice sheets rapidly melted.
For additional information see Snowball Earth by Hoffman and Schrag, from the January 2000 issue of Scientific American.
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